Breeding

Pair Formation

Probably remain in pairs throughout migration and winter, but not documented. In Middle Atlantic states, arrive from late Apr to early May (Gochfeld 1978a, Erwin 1979), and large dense flocks settle on potential nesting areas in Common Tern colonies about 1–2 wk after terns arrive. In se. U.S., breeding begins mainly in May, but as early as mid-Mar in Texas (Clapp et al. 1983). At arrival, pairs are not apparent in dense flocks, but after a week members of pair stand closer to each other than to neighboring pairs. After 10–14 d the birds separate into subcolonies of a few to about 100 pairs, and these gradually spread out to occupy territories in the most open habitats. Both sexes make scrapes using exaggerated sand-kicking posture; often several scrapes in one territory; 1 chosen as nest.

Egg-Laying

See Figure 5 . In New York, 31 May–3 Sep (Bull 1974). Egg-laying is more highly synchronized within subcolonies than among subcolonies, but large subcolonies are no earlier or more synchronous than small ones (Gochfeld 1979a). More than 80% of first nests are initiated by mid-Jun (New York). In most years, peak hatching by late Jun, fledging by mid-Aug. In Maryland, nesting peak mid-Jun to late Jul (Stewart and Robbins 1958). In Virginia peak egg-laying in 3rd wk of May (Erwin 1977a). In S. Carolina, start nesting in late May, continue to early Sep (Blus and Stafford 1980). In Florida, present throughout year, breeding May–Sep (Kale and Maehr 1989). In Louisiana (1930s), described as a late breeder, with egg-laying mainly in Jun and fledging in Aug (Oberholser 1938). On Gulf Coast, peak of incubation in early Jul (Portnoy 1977).

Second Broods

No evidence of raising a second brood; if nest fails, however, will relay up to 3 times, and in some years fledging delayed to Oct (such late young probably do not survive) (MG unpubl. data).

Most colonies on beaches, bars, dredge deposition, salt marsh (Bent 1921, Frohling 1965), and rooftops (in Florida since 1986) (Gore 1991). More than 75% of the New Jersey salt marsh colonies are on islands 0.25–10 ha in size. In Louisiana, 64% nested on barrier beach, 19% on shell bars, and 17% on dredge deposition islands (Portnoy 1978b). In New Jersey, species avoids islands < 0.5 ha and > 20 ha; prefers marshes with > 20% Spartina patens and avoids islands with only S. alterniflora . Prefers beaches with < 20% vegetation (e.g., Cakile edentula, Solidago sempervirens, Ammophila breviligulata), and often nests with no vegetation at all (Burger and Gochfeld 1990). Of 32 Long Island, NY sites, 19% on dredge deposition islands and 80% on beaches. In w. Florida, 2 of 12 colonies on beaches, 7 on roofs, and 3 on disturbed land (dredge spoil, land cleared for construction) (Gore 1991).

On Long Island, NY, < 1% “nestings” without terns. In 1977, all skimmers nested in tern colonies and 92% were in colonies with > 1,000 pairs of Common Terns, while only 1 large tern colony lacked skimmers. Numbers of terns and skimmers positively correlated (r = + 0.91) (Gochfeld 1978a). New Jersey skimmers also nest with Common Terns (Stone 1937, Burger and Gochfeld 1990), and monospecific colonies are rare. In w. Florida, 9 skimmer colonies included Least Terns (Sterna antillarum) while 3 were monospecific (Gore 1991). In Texas 10% of nestings involved only skimmers, while Laughing Gulls, Gull-billed (S. nilotica), Least, or Forster’s Terns were associated at about 30% of colonies (Texas Colonial Bird Survey 1982).

Selection

Historically, most colonies on sandy beaches or islands, some on shell bars and probably on salt marshes. In past 50 yr, increased use of dredge deposition islands (particularly in Carolinas) (Tomkins 1933, Parnell and Soots 1980), salt marshes (particularly in New Jersey), and rooftops (in Florida; Kale and Maehr 1989). Select future colony sites by settling there in flocks.

Site Characteristics

Skimmers prefer open, sandy substrate, even on salt marsh islands when present, but will nest on mats of dead vegetation (seawrack). Vegetation cover around nests averaged 3.2% versus 14% around Common Tern nests in same quadrat (p < 0.001) (Gochfeld 1978a); nest further from grass or bushes than random (Burger and Gochfeld 1990) and will nest on completely barren beaches, although some vegetative cover around nesting area may be preferred; tend to occupy areas with up to 10% vegetation and to shun areas with > 30% cover. On salt marsh, nest only on mat of dead vegetation or on sandy areas, not on bent over salt marsh grasses. Nearly white eggs camouflaged well on pale mats (Burger and Gochfeld 1990). Eggs and chicks better camouflaged on sand, which is preferred almost everywhere else in its range. Usually nest closer to skimmers than to terns, and closer to center of mat than to edge; will displace terns from the center of mats (Burger and Gochfeld 1990).

On salt marsh islands in New York and New Jersey, nest on mats of dead Spartina or Zostera piled on the marshes. Experimental placements of mats on New Jersey salt marshes revealed preference for mats > 2 m wide; avoid islands with narrower mats or none at all (Burger and Gochfeld 1990).

Common Terns and other tern species are an important feature of skimmer colonies. On Long Island, NY, skimmers nested only with Common Terns (even if only 1 tern nest present) (Buckley et al. 1978). Nest mainly with Common Terns in New Jersey and Virginia (Erwin 1977a, Burger and Gochfeld 1990), Gull-billed Terns in S. Carolina (Blus and Stafford 1980), Least Terns in Florida (Kale and Maehr 1989), Forster’s, Least, and/or Gull-billed Terns, and Laughing Gulls along Gulf Coast (Clapp et al. 1983), Large-billed Tern (Phaetusa simplex) and/or Yellow-billed Terns (Sterna superciliaris) in South America (Murphy 1936, Preston 1962, Groom 1992, Krannitz 1989). In Louisiana, however, may form large monospecific colonies (Oberholser 1938).

Of 297 salt marsh nests (in New Jersey 1979), the half that were more centrally located were initiated 9 d earlier, were more synchronous, had larger clutch, greater density, and were closer to terns and had slightly higher hatching success than the half located closer to mat edges (Burger and Gochfeld 1990). Some beach colonies were sufficiently high to avoid flooding. Heavy rain and spring tides regularly flood salt marsh colonies. Some nests may survive on floating mat or if contact with water limited to few hours.

Construction

Mates take turns scraping, using exaggerated sand-kicking posture (neck, head, bill, and tail elevated) with alternate foot strokes that throw sand backwards (Burger and Gochfeld 1990). Birds rotate in scrape and create a saucer-shaped depression, similar to resting scrapes used throughout the year. The depression takes only a few minutes to create, but the process of nesting may involve several scrapes and nest-showing behavior, requiring 5–7 d between onset of nest “building” and laying. Males do more scraping and make larger scrapes than females.

Structure, Composition, And Dimensions

Nest a scrape in the sand or a depression in salt marsh mats, about 2–4 cm deep and 20–30 cm in diameter. No material is added to the nest.

Microclimate

Nest rarely has any shade; virtually all nests are in the open, usually at least 15 cm away from vertical objects; thus all nests subject to full insolation. Even as far north as Long Island, NY, sand surface temperature on hot sunny afternoons reaches 55°C. Chicks may perish within 25 min of full solar exposure.

Grant et al. (1984) studied nest microclimate. Nest vapor pressure in 4 North Carolina nests 25.3 torr compared to 20.2 torr ambient. Water vapor conductance 6.4 mg/day torr. Water loss of 24 eggs averaged 143 mg/day totalling about 11%. Incubation temperature of 6 eggs 35.6°C.

Maintenance Of Nests

Throughout incubation, nests are maintained by sideways settling behavior and sand-kicking. Chicks remain in the nest for 1–5 d post-hatching. Thereafter new scrapes are formed by young chicks and by adults brooding them.

Shape

Mainly rounded-ovate to elongated-ovate; 4th egg shortest in 64% of 43 clutches and narrowest (or tied) in 61%. Egg shape apparently consistent within females (MG unpubl. data).

Size

See Table 1 . Egg sizes show a left skew (median > mean). Length and width are positively correlated for all eggs in a clutch, particularly the first (r = +0.86, p < 0.0001, n = 73) (MG unpubl. data).

Mass

Mean fresh mass of 12 N. Carolina eggs averaged 26.9 g (Grant et al. 1984). Each egg approximates 8–10% of female body mass. First eggs usually with greatest width and brownest pigmentation. Last egg smallest and palest in 89% of clutches (Burger and Gochfeld 1990).

Color

Ground color pale cream, ivory, to white; rarely slightly greenish or pinkish; blotched and spotted with intricate patterns of dark brown to black. First eggs have slightly but consistently darker ground color and heavier markings than subsequent eggs and last eggs often strikingly paler and less marked.

Surface Texture

Smooth, usually non-glossy; freshly laid eggs have adherent grains of sand until incubated.

Eggshell Thickness

Shells averaged 5,500 pores with a functional pore area of 0.61 mm²; shell density 1.8 h/cm³ (Grant et al. 1984). Blus and Stafford (1980) found no shell thinning in S. Carolina, but White et al. (1984) found up to 12% thinning in Texas skimmer colonies while King et al. (1991) found only 5% thinning compared with pre-1943 thickness. In 1970s, about 0.5% of skimmer eggs on w. Long Island, NY, were shell-less yolks laid outside of nests. Frequency declined to < 0.1% by late 1980s.

We collected shells of hatched eggs from Cedar Beach, NY, and Barnegat Bay, NJ, for several seasons in the early 1970s, 1980s, and 1990s. In New York, thicknesses averaged 0.366 mm ± 0.010 (n = 31), 0.363 mm ± 0.01 (n = 45), and 0.546 mm ± 0.01 (n = 49) for those three periods, respectively (MG unpubl. data). For New Jersey, 0.350 mm ± 0.02 (n = 16) and 0.424 mm ± 0.01 (n = 3) for the early 1980s and 1990s, respectively (JB unpubl. data). Skimmer eggshells increased in thickness between 1980 and 1990, perhaps reflecting declining DDE residues in the population. Other measurements (without shell membranes) are not directly comparable. Shell thickness (without membranes) averaged 0.25 mm prior to 1943 and 0.24 in the early 1970s and 1980s (King et al. 1991).

Egg-Laying

Begins 7–10 d after first nest scrapes appear. Eggs laid at 1–2 d intervals, and A and B eggs occasionally on same day. Most eggs laid before 1000 h;. 4-egg clutch usually completed in 4–6 d (Erwin 1977a).

Synchrony Of Clutch Initiation

At Cedar Beach, NY, clusters or subcolonies were more synchronous than the colony as a whole (Gochfeld 1979a); larger subcolonies tended to have later laying dates, consistent with continued recruitment through the season. Erwin (1977a) was impressed that clutch initiation was asynchronous.

Onset Of Broodiness And Incubation In Relation To Laying

Incubation begins with laying of first egg, resulting in asynchronous hatching.

Incubation Patch

Both sexes have incubation patches.

Incubation Period

Can be 21–23 d, but usually 23–25 d due to disturbance. Averaged 22.9 d at 110 Virginia nests (Erwin 1977a) and 24.3 d at 246 nests in New York (MG, JB unpubl. data).

Parental Behavior

At undisturbed nests, eggs are incubated constantly and are uncovered only momentarily during nest changes. Both sexes incubate (contra Tomkins 1933). Females incubated more than males in Texas (Quinn 1990), but in New York males performed about 55% of incubation and 65% of chick brooding and were more likely to be away from colony when female was incubating or brooding. Females spent 35% of non-incubating time and 16% of non-brooding time standing near the nest. In the Northeast, males present 74% and females 79% of daylight hours during incubation (Burger 1981a); thus for most of the day and night both members of the pair are present. In Texas males present up to 77% and females 82% of hourly censuses (Quinn 1990). Birds protect nests from drifting or wind-driven sand by frequent sideways-settling and sand-kicking movements; readily retrieve displaced eggs and even uncover eggs buried in nest (Gochfeld 1978a). Mates exchange about every 2–4 h; male sometimes pushes female off the nest.

Temperature And Water Tolerance

Eggs tolerate chilling during earliest days of incubation, but thereafter vulnerable to overheating or chilling. Prolonged exposure to sun is lethal and parents incubate tenaciously on hot afternoons. Tolerate immersion in freshwater for 24 h and in saltwater for few hours (exposure > 6 h lethal).

Preliminary Events And Vocalizations

Starlike pattern of cracks appears on large end of egg about 48 h prior to hatching. Pipping occurs about 24 h prior to hatching, and by this time faint cheeping calls will be heard if the egg is manipulated.

Shell Breaking And Emergence

Young enlarge initial pip with egg tooth and with vigorous neck extensions. From pipping to hatching requires about 24 h, and if interval prolonged beyond 36 h hatching failure is likely to occur (MG unpubl. data). If chilling occurs, a pipped egg may fail to hatch. Hatching times distributed throughout day with no data to suggest temporal peaks (MG, JB).

Intervals And Synchrony

Eggs in a typical 4-egg clutch usually hatch at 1 d intervals, although interval from 3rd to 4th egg prolonged. Last egg in clutch fails to hatch in about 10–15% of nests, if adults accompany earlier chicks and cease incubation.

Parental Assistance And Disposal Of Eggshells

Parents remove hatched shells, usually within 30 min of hatching. Shells usually carried in flight and dropped outside of territory.

Condition At Hatching

Semi-precocial chicks are completely covered with down, although at hatching down is wet and chick appears mostly naked, but dries after about 2–3 h of brooding. Chicks can open eyes at hatching and, although uncoordinated, can walk from nest within 1–2 h, although actual walking not usually seen until 2nd day. Usually spend most of time sleeping under parent. Mandibles equal in length. Chick completely dependent on parent for food until after fledging, but begin to mandibulate inedible objects picked up from surface by about 3–4 d of age.

At hatching, bill and feet dusky pink. Bill has blackish tip. Legs become more pink by about 1 wk of age, remain so through the first winter. The eye is dark brownish black. Small egg tooth not apparent after 4th day.

Growth And Development

See Erwin (1977a) for details. Mass increases slowly in first 2 d then shows rapid spurt until about 3 wk of age. Body parts grow at different rates. Pin feathers of primaries appear about day 5. Sexual dimorphism readily apparent by 17 d, with many males exceeding 300 g by that time (adult females rarely reach 300 g) (MG). Wing length grows linearly after day 7, and in 7 chicks age = 0.109 x wing length (mm) + 5.40. At fledging, 11 males averaged 357 g and 10 females averaged 260 g. Growth data fit a Richard’s model with asymptote of 366.4 (males [M]) and 270.9 (females [F]); growth constants of 0.274 (M) and 0.289 (F), 14.25 d to inflection (M) versus 11.73 (F), and a shape constant of 2.40 (M) and 2.32 (F) (Schew and Collins 1990). At fledging, 5 Virginia males averaged 295.2 compared with 264.4 for 7 females (Erwin 1977a).

During 1st wk of life, nestlings vulnerable to overheating if parents kept off nests and experience high mortality from exposure, starvation, or aggression by neighboring terns or skimmers; brooded most of the time unless disturbed. During 2nd wk, brooded for shorter periods and spend much time standing in the open or crouched in exposed scrapes, where they are vulnerable to prolonged rain and cold.

If undisturbed, newly hatched chicks lie in nest, often with necks overlapping; siblings remain in contact until they begin to wander from nest at 3–5 d of age. Typically in salt marsh colonies where the surrounding marsh grass is unsuitable habitat, chicks remain within 1–2 m of the nest until fledging if not disturbed, and often return to the natal territory following disturbance. At beach colonies, following disturbance, siblings often found in separate self-made scrapes and may leave territory. Using sideways-settling movement, young birds create a new scrape which enhances ability to hide (Hays and Donaldson 1970) and moderates temperature.

By age 2 wk, run rapidly and can almost elude human pursuers; may run 200 m along beach without seeking cover. Flight develops gradually. Birds in 4th week jump and flap. Initial flight is into the wind, and birds are sometimes carried far from their territory. Some young flying by 28–30 d (Erwin 1977a, Burger and Gochfeld 1990). Prior to first flight, birds run long distances from intruder, but afterward take flight immediately and never run again. The smaller and lighter females fly about 1–3 d before males. With frequent or excessive disturbance, young may run long distances, or adults may call and coax them to safer areas; e.g., one 1-wk-old brood was found 1.4 km down the beach from its nest site (Gochfeld 1981b). Chicks > 2 wk stand on territory, perhaps in shade of bush, and run out when parents return with fish.

Adults and young gular flutter and crouch in fresh scrape on sand when heat-stressed. Young may seek shade. In s. U.S., adults may wet belly feathers to moisten eggs (not adequately documented, but this occurs regularly in African Skimmer) (Roberts 1976). Hatchling exposed to full sun without shelter died in about 25 min. Adults and young flatten themselves onto sand, extending wings to maximize heat loss and reduce radiant uptake from hot sand. On cold rainy days, unbrooded chicks suffer hypothermia; particularly 7- to 15-d-old chicks that are no longer regularly brooded. Very high mortality often seen in this age group while younger, brooded chicks survive (Burger and Gochfeld 1990); prolonged (40 h) storm on 4 Jul 1978 killed 100% of skimmer chicks on Long Island, NY, and New Jersey.

Brooding

Hatching asynchronous; brooding begins immediately upon hatching. Chicks brooded most of the time during the 1st wk , guarded for the next 2 wk. Chicks older than 1 wk vulnerable to storms because they are too large to be brooded. In Texas, females incubated more, but did not brood more, than males (Quinn 1990); in New York, males brooded more than females at 9 of 11 nests (Burger 1981a), but females brought back more fish for young and fed them more, particularly at < 5 d of age. For pairs with chicks, females performed more aggressive behavior than males, but after nest failure, males resumed more of the aggression. Parents sometimes divide brood duties, and in many cases would each brood chicks in adjacent scrapes (Burger 1981b). Brood tends to remain in close proximity to each other; both parents attend young.

Feeding

Both parents bring fish to young. Adult lands on territory with fish and approaches chicks in nest, uttering soft call notes. Day-old chicks remain crouched and may require coaxing to take fish from parent’s bill, but older chicks beg vigorously even before parents land, and after about 3 d run towards parent. Siblings compete for fish; older chicks may monopolize parents, although some parents appear to feed younger chicks selectively, a behavior that warrants study. If chicks drop fish they usually ignore it, although parents will pick it up and re-feed or swallow it themselves. In 86 h of observation in Virginia, average of 0.43 feedings/young/h, with an average of 14.6 min between feedings (Erwin 1977a).

Young chicks were fed fish < 5 cm long, usually Fundulus, with larger fish brought to older chicks (Burger and Gochfeld 1990). Quinn (1990) reported males bringing back larger fish than females.

Nest Sanitation

Eggshells removed within 30 min of hatching and dropped 2–20 m away. No sanitation. Young defecate in or out of nest. No specific invertebrate associates, but in some places ants invade nests and kill hatchlings. Parents may carry a dead chick away from the territory (M. Erwin pers. comm.), and occasionally pick up, carry, kill, or even eat intruding young skimmers or terns (Burger 1981b, Ali and Ripley 1983, Safina and Burger 1983).

Parental Carrying Of Young

African Skimmer reported to carry chick away when threatened (Roberts 1976); but never observed in Black Skimmer, although we have seen skimmers seize, fly away with, and even swallow an intruding skimmer or tern chick. Groves (1977) reports skimmers retrieving displaced chicks in Florida, and, in s. New Jersey D. Weidner (pers. comm.) saw adults retrieve chicks dropped by Laughing Gulls, catching the chicks in midair, and dropping them on the nest territory; the chicks survived.

Parental care persists at least several weeks after young capable of flight (Erwin 1977a).

No information, must be very rare if it occurs. No reports of helpers.

No reports of brood parasitism.

Fledglings tend to join flocks of nonbreeding or postbreeding adults on beach and spend hours standing or lying. They make first skimming flights within 2 d of first flight, but appear to have low success initially, and are dependent on parents for at least 2 wk, probably much longer. Juveniles are present with adults in wintering flocks.